1. Field of the Invention
The present invention relates to an injection-locked discharge excited laser device for exposure apparatus, comprising a narrow-band laser oscillation stage laser and an amplification stage laser, and more particularly to a laser device for exposure apparatus that allows to extend the life of optical elements such as chamber windows, output coupling mirrors (OC) and the like.
2. Description of the Related Art
Excimer lasers are being used in recent years as light sources for semiconductor exposure devices. In particular, ArF laser light sources having high outputs (40 W or more) and ultra-narrow bands (0.2 pm or less) are being used in the technology node (not more than 45-nm).
With a view to securing high dose stability and achieving higher throughputs, yet higher outputs, of 90 W or more, are being required of ArF laser light sources in light sources for exposure devices.
To meet the above demands as light sources, double-chamber (two-stage) ArF lasers are being used in practice. Broadly, a double chamber laser device can be embodied as a MOPA (Master Oscillator Power Amplifier) device, in which there is provided no resonator mirror at the amplification stage, or as a MOPO (Master Oscillator Power Oscillator) device, in which there is provided a resonator mirror.
To achieve high outputs such as 90 W, the load on the chamber window of the amplifier (PA) or, the optical elements (in particular, chamber windows and OC) of the amplification stage laser (PO), increases, which is problematic in terms of the life of these optical elements. It has become therefore necessary to prolong the life of laser light sources.
Japanese Translation of PCT Application No. 2005-524998 and Japanese Unexamined Patent Application Laid-open No. 2006-049839, for instance, disclose the following technologies for extending the life of an optical element.
Japanese Translation of PCT Application No. 2005-524998 discloses, as illustrated in FIG. 8, a MOPA laser device having an oscillation stage laser (MO) 100 and an amplifier (PA) 200, with a prism beam expander 201 arranged between a discharge electrode of the amplifier (PA) 200 and a laser chamber window of the amplifier (PA) 200. Such a configuration reduces the load (energy density) in the laser chamber window of the amplifier (PA) 200.
Japanese Unexamined Patent Application Laid-open No. 2006-049839 describes a MOPO laser device having an oscillation stage laser (MO) 100 and an amplification stage laser (PO) 300, wherein a beam expander 302 is arranged between a laser chamber 301 and an output coupling mirror (OC) 303 of the amplification stage laser (PO) 300, as illustrated in FIG. 9. Such a configuration reduces the load (energy density) in the output coupling mirror (OC) 303.
When in the above MOPO laser device a ring resonator having high seed injection efficiency is used in the amplification stage laser (PO), the OC and the windows of the chamber of the amplification stage laser (PO) are subjected to a high load, at a final output of 90 W (15 mJ, 6 kHz), which shortens the life of these elements. FIGS. 10A and 10B illustrate a conventional example of a MOPO-type laser device having a ring resonator arranged in the amplification stage laser (PO). FIG. 10A is a side-view of the amplification stage laser (PO) 20, while FIG. 10B is a top-side view of the same.
In the figure, a laser beam emitted by an oscillation stage laser (MO) 10 functions as a seed laser beam, while the amplification stage laser (PO) 20 has a function of amplifying that seed laser light. The oscillation stage laser (MO) 10 and the amplification stage laser (PO) 20 have each respective laser chambers 11, 21, the interior whereof is filled with a laser gas. Inside each laser chamber there is arranged a pair of opposing electrodes 1a, 2a separated by a predetermined distance, such that discharge is effected through application of high-voltage pulses to these pairs of electrodes 1a, 2a. 
In the chambers 11, 21 of the oscillation stage laser 10 and the amplification stage laser 20 there are arranged, respectively, window members 12a, 12b, 22a, 22b manufactured using a material having transmissivity towards laser oscillation light.
The oscillation stage laser 10 has a line-narrowing module (LNM) 3 that comprises an expanding prism 3a and a grating (diffraction grating) 3b, such that the optical elements in the line-narrowing module 3 and an OC 14 constitute a laser resonator.
MO laser light (seed laser beam) from the oscillation stage laser 10 is guided via high reflection mirrors 4a, 4b, 4c and is injected into the amplification stage laser (PO) 20.
As illustrated in FIG. 10B, the amplification stage laser (PO) 20 has provided therein a ring resonator comprising an OC 24, which is a partial reflection (PR) mirror having an antireflective (AR) film on the light-incidence side, and high reflection mirrors 5a, 5b, 5c. 
A beam outputted from the oscillation stage laser (MO) 10 is injected by the high reflection mirrors 4a, 4b, 4c into the OC 24 of the ring resonator of the amplification stage laser (PO) 20. A high reflection mirror 5a causes the beam to pass through a discharge-free space in a laser chamber 21, and high reflection mirrors 5b and 5c bend the beam into a discharge electrode space. Discharge is carried out through application of voltage between the discharge electrodes 2a, synchronizing with the seed light. The seed light passing through the discharge space is thus amplified, then part of the amplified light passes through the OC 24 and is outputted as a laser, while light reflected by the OC 24 resonates through feedback once more in the ring resonator. Output is effected thus in the form of laser pulses. When the reflectance of the OC 24 is, for instance, of 20 to 30%, some 70 to 80% of the beam outputted from the MO becomes injected into the ring resonator, which allows achieving high injection efficiency. In the case of a MOPA laser device, there is no light resonator in the amplification stage and no resonation-amplification is carried out, so that the device functions merely as an amplifier. As a result, the MO output must be about 10 times that of a MOPO laser device comprising a ring resonator.
When using a ring resonator in the amplification stage laser (PO) 20, however, the load inside the ring resonator increases extraordinarily when the output of the laser is high (90 W or more). In particular, the partial reflection (PR) film that coats the OC 24 of the amplification stage laser (PO) 20 was damaged, which shortened the life thereof. The life of the output-side window 22a of the amplification stage laser (PO) 20 was also shortened on account of high energy density.
Thus, as described in Japanese Translation of PCT Application No. 2005-524998, an optical element for beam expansion may conceivably be arranged on the optical path of a ring resonator provided before the optical element susceptible of deterioration. FIG. 11 illustrates an example of the amplification stage laser (PO) 20 having such a configuration.
In FIG. 11, specifically, a beam expander 7, which is an optical element for beam expansion, is arranged on the optical path of the ring resonator before the output-side window 22a. 
When a beam-expanding optical element is arranged in the optical path of the ring resonator, as illustrated in FIG. 11, the ring resonator becomes an unstable-type resonator (a resonator in which a beam is expanded each time it makes a round trip through the resonator). This gives rise to the following problems.                Large oscillation losses in the amplification stage laser (PO) and impaired injection efficiency.        Higher spatial coherence of the output laser light, resulting in generating speckle pattern on the mask of an exposure device.        